Freshly mixed asphalt paving material is usually transferred from a pug mill or mixing drum to a storage bin or silo in which it is held until it is transported to a job site by truck. Storage of material in the bin allows the mixing and the trucking operations to be conducted on independent schedules.
The storage bin or silo is typically in the form of an upright generally cylindrical vessel having a conical bottom portion that converges down to an outlet controlled by a gate. The vessel is mounted to have its outlet high enough so that a truck can be driven under it to be loaded with material issuing directly from the outlet.
The gate that controls the silo outlet should be capable of opening and closing rather rapidly, so that the flow of material into a truck can be started and stopped quickly. The gate must also be capable of supporting a substantial vertical load, because when it is closed a portion of the weight of the material in the silo rests upon it.
In addition to these requirements, it is now mandatory in many areas that there be provision for a substantially airtight seal at the outlet of an asphalt mix storage bin if asphalt mix to be stored in it overnight or for other extended periods. The need for such a seal arises because asphalt paving mix must be kept at a temperature above 300.degree. F. (over 140.degree. C.) if it is to be prevented from hardening. To prevent cooling of the contents of an asphalt mix silo, such a silo is thermally insulated. But if there is any substantial air leakage at the bottom outlet of a silo, there could be a strong upflow of air through its interior, induced by the heat of its contents and the chimney effect of the silo walls, and such airflow could carry off enough heat to permit substantial hardening of the mix, especially in the downwardly tapering bottom portion of the silo through which the coolest air would flow. Needless to say, a plug of hardened asphaltic material near the bottom outlet of a silo would present a difficult and unpleasant problem.
There is some belief that for long-term storage of asphalt mix air should be purged out of the interior of the silo and totally excluded from it, on the theory that the asphaltic binder component of the mix tends to be oxidized in the presence of air, with consequent deterioration of its quality. On this theory, U.S. Pat. No. 3,348,739 discloses means for forcing pressurized inert gas into an asphalt mix silo to drive air out of it and maintain a nonoxidizing environment in its interior; and U.S. Pat. No. 3,820,687 discloses the circulation of air from and back to the silo interior through a charcoal burner by which the oxygen content of the air is converted to carbon dioxide.
There are indications, however, that little or no deleterious oxidation takes place during reasonably long term storage, provided the bottom outlet of the silo is sealed substantially airtight. Apparently such gases as are given off by the asphaltic binder material tend to displace residual air out of the silo interior, especially if the top inlet is not sealed airtight. If such gases are substantially inert, then the upper seal would not have to be particularly good, inasmuch as air does not tend to flow downwardly through it into the hot interior of the silo.
It is apparent, however, that a substantially airtight bottom outlet closure is essential for a silo in which asphalt mix is to be stored for long periods, whether or not possible oxidation of the binder material is a matter of concern. But the need for such a seal has heretofore been regarded as somewhat incompatible with the requirement that the closure gate for the bottom outlet be capable of rapid opening and closing, and also with the requirement that the gate be capable of supporting a substantial load when closed. Of course a completely satisfactory sealing closure for the bottom outlet should also be inexpensive and easy to manufacture and should require a minimum of maintenance.
One type of bottom closure for asphalt mix storage bins that has heretofore been devised in an effort to meet this complex of requirements is disclosed in U.S. Pat. No. 3,532,252, wherein the bottom outlet was normally closed by a pair of clamshell gates that swung toward and from one another. Around and beneath the outlet was a box-like structure that cooperated with the frustoconical bottom wall portion of the silo to form a chamber beneath the outlet. This chamber had a bottom opening that was aligned with the silo outlet and could be sealed closed by a second gate in the nature of a sliding door. The chamber and the silo were intended to be filled with inert gas; hence leakage through the clamshell gates was of no consequence inasmuch as the chamber beneath those gates served as a sort of air lock. Obviously it was expensive to provide the box-like chamber structure and the two sets of gates, and in addition the patent discloses a rather complicated control system, apparently needed to ensure, among other things, that the clamshell gates would not be opened while the lower sliding door gate was still closed.
A later arrangement for sealing a bottom outlet is disclosed in U.S. Pat. No. 3,949,907. The closure of that patent comprised a large clam shell gate that swung in an arc between open and closed positions. When the gate was closed, marginal portions of it extended upwardly and outwardly all around a downwardly projecting rim around the silo outlet, to cooperate with that rim in defining an annular trough around the outlet. If the gate was to remain closed for a substantially long time, oil was filled into this trough to provide a seal. The oil was drained out just before the gate was to be opened. The gate structure just described, in comprising an arcuately swinging gate member, required a curved bottom surface on the rim of the outlet, corresponding to the arc of swinging motion of the gate, and required the gate to be curved concentrically to its path of motion. The structure was therefore difficult and expensive to fabricate, and a somewhat complicated actuating mechanism was required for imparting swinging motion to the gate. Furthermore, because of the large size of the single swingable gate member, needed for defining the oil seal trough, it possessed substantial inertia and therefore was not well adapted for rapid opening and closing.
The earlier U.S. Pat. No. 3,532,252, in addition to its air-lock bottom closure seal, also disclosed a sealing closure for the top inlet. That closure comprised a flat, plate-like rectangular door which was edgewise slidable to and from a closed position over the silo inlet and which carried four small single-action pneumatic cylinder motors, one near each of its corners, each arranged to compress a strong coiled expansion spring. As the door was moved to and from its closed position, the pneumatic motors were energized to hold the springs compressed; but with the door in its closed position, air pressure on the motors could be relieved, and the springs would then expand, reacting against fixed structure on the silo to bias the door flatwise downward into firm engagement with the rim of the inlet.
A later U.S. Pat. No. 3,946,772 disclosed another slidable gate type of inlet closure, requiring only one pneumatic cylinder motor, arranged to impart edgewise sliding motion to the gate in its opening and closing directions. A rather complicated system of rollers, cam tracks and toggle links was relied upon to move the gate flatwise downward into firm engagement with the inlet rim after the gate had been moved edgewise into a position overlying the inlet opening.
It is noteworthy that the two sliding gate arrangements just described were intended for silo inlets. An inlet closure gate does not have to move into downwardly flowing asphalt mix and interrupt its flow, as with a conventional silo outlet closure; hence, both the gate itself and the rim of the inlet can be expected to remain reasonably clean. If particles of sand or the like can lodge between the gate member and the closure rim, they naturally maintain a space between those elements that prevents the attainment of an airtight seal.
U.S. Pat. No. 3,348,739, which taught the charging of pressurized inert gas into an asphalt silo, also disclosed an edgewise slidable gate for the silo bottom outlet, guided in opposite grooves in the fixed silo structure and intended to make wedging engagement in another groove when in its closed position. In that case the sliding gate did not have to move into the path of flowing asphalt mix because flow through the outlet was controlled by a worm conveyor that moved the mix substantially horizontally to the outlet. Nevertheless, it is doubtful whether the gate guiding grooves could have been kept sufficiently clean and free from asphalt mix to ensure consistent and troublefree operation of the closure.
The above discussed prior art demonstrates that the provision of a fully satisfactory sealing closure for an asphalt mix silo outlet has been far from obvious. The complicating factor is one that appears to be unique to closures for asphalt mix silos, namely the presence of asphalt mix, which is both sticky and gritty and the gritty particles of which, moreover, are extremely hard. Because of this complicating factor, the attainment of an airtight seal for an asphalt mix silo outlet has been thought to require structures, actuating mechanisms and control systems that were complicated and expensive.